High-frequency spectrum analyzers typically include a series of frequency conversion stages to facilitate analysis of applied input signals. In FIG. 1, a block diagram of frequency conversion stages within a conventional high-frequency spectrum analyzer are shown. An input signal SIN at frequency fIN that is applied to the spectrum analyzer is converted to a first intermediate frequency signal IF1 by mixing the input signal with a signal SLO1 at frequency fLO1 provided by a first local oscillator LO1. This first intermediate signal IF1 is further converted to intermediate frequency signals IF2, IF3 having successively lower frequencies fIF2, fIF3, respectively. Bandpass filters BPF1-BPF3 eliminate image signals resulting from the mixing to provide an unambiguous representation of the applied input signal SIN at the third intermediate frequency fIF3.
The third intermediate frequency signal has a frequency fIF3=fIF2xe2x88x92fLO3=fIF1xe2x88x92fLO2xe2x88x92fLO3=fLO1xe2x88x92fINxe2x88x92fLO2xe2x88x92fLO3, indicating that a detected response at the frequency fIF3 is attributable to the input signal SIN having a frequency fIN=fLO1xe2x88x92fLO2xe2x88x92fLO3xe2x88x92fIF3. Because the frequency fIF3 of the third intermediate frequency signal IF3 and the frequencies fLO1, fLO2, fLO3 of the local oscillators are known, the frequency fIN of the input signal SIN is readily established. The frequencies fLO2, fLO3, and fIF3 are generally fixed, whereas the frequency fLO1 of the first local oscillator LO1 is generally tuned to accommodate input signals SIN over a wide range of frequencies fIN.
The measurement accuracy of a spectrum analyzer depends on the quality of the signals SLO1-SLO3 provided by the local oscillators LO1-LO3 in the frequency conversion stages. Ideally, the local oscillators provide stable, low-noise signals and don""t contribute significant noise to the intermediate frequency signals provided by the frequency conversion stages. However, practical implementations of the local oscillators provide signals that have short-term frequency instabilities and fluctuations, commonly referred to as phase noise. Since the phase noise of a local oscillator generally increases with operating frequency, the first local oscillator LO1, being the highest frequency local oscillator in the spectrum analyzer, is typically a higher phase noise contributor than the other local oscillators in the spectrum analyzer. In addition, the frequency tuning feature of the first local oscillator LO1 tends to further increase the phase noise the local oscillator. Because the phase noise of the local oscillators can degrade the measurement accuracy of the spectrum analyzer, there is motivation to improve the noise performance of the local oscillators in the frequency conversion stages of the spectrum analyzer, particularly by lowering the phase noise of the first local oscillator LO1.
A known way of lowering phase noise involves using an offset-loop synthesizer 2 to generate the first local oscillator signal SLO1, as shown in FIG. 2. The offset-loop synthesizer includes a course-step synthesizer phase locked loop (PLL) 4 that provides an offset signal S0 to a main (PLL) 6 of the offset loop synthesizer that provides the signal SLO1. The offset signal S0 eliminates the need for frequency division in the feedback path of the main PLL, thereby reducing the phase noise of the signal SLO1. However, because the frequency f0 of the offset signal S0 is multiplied by a harmonic mixer 40 in the main PLL, the phase noise of the offset signal S0 is also multiplied. Accordingly, it is advantageous for the offset signal S0 provided by the coarse step synthesizer PLL to have especially low phase noise.
Low phase noise is typically achieved in the coarse step synthesizer PLL by eliminating frequency division in the feedback path in the M/N loop used to implement the coarse step synthesizer PLL. This is done by setting the value of M to unity and by varying N, the divide ratio of the programmable divider D, to set the frequency f0 of the offset signal S0. The harmonic mixer in the main PLL uses the H-th harmonic of the signal S0 to produce a signal with a frequency close to the resulting frequency fLO1 of the signal SLO1, whereas an interpolation signal SINT provides fine frequency resolution for the signal SLO1. This results in the signal SLO1 having a frequency fLO1=H*foxc2x1fINT, where fINT is the frequency of the interpolation signal SINT. This frequency relationship illustrates that the phase noise of the signal SLO1 is the harmonic number H times the phase noise of the coarse step synthesizer PLL, plus the phase noise of the interpolation signal. Thus, while the noise gain of the main PLL with respect to the interpolation signal SINT is unity, the noise gain with respect to the coarse step synthesizer PLL is H, the harmonic multiplier of the harmonic mixer in the main PLL of the offset loop synthesizer. This noise gain results in multiplication of the inherent phase noise of the VCO and other components of the coarse step synthesizer PLL, which can degrade the measurement accuracy of the spectrum analyzer within which this type of offset loop synthesizer is included.
Embodiments of the present invention are directed toward a direct frequency synthesizer suitable for replacing the coarse step synthesizer PLL in an offset loop synthesizer. The output signal from the direct frequency synthesizer is derived from a high frequency reference signal that is frequency divided and mixed to satisfy the coarse step synthesis requirements of an offset loop synthesizer. The absence of a VCO within the direct frequency synthesizer provides the direct frequency synthesizer with lower phase noise than a typical PLL-based coarse step synthesizer. Though applicable to a variety of types of synthesizers and signal generators, the direct frequency synthesizer can provide especially low phase noise when used to generate an offset signal for an offset loop synthesizer of the first local oscillator of a spectrum analyzer, where the second local oscillator of the spectrum analyzer provides the reference signal for the direct frequency synthesizer. Alternative embodiments of the present invention are directed toward a direct frequency synthesis method.